Being a monk, the Venerable Bede would have felt like taking a break from music to go and do some writing!
The primary duty of medieval monks (a gender-neutral term in Bede's day!) was to pray in community the Divine Office, a set of prayers prescribed for specific times of the day and night. (Fancy a 3 AM-til-sunrise plainchant marathon?) The central text for the lyrics was the Book of Psalms, with supplements from hymns and other miscellaneous religious texts. The monks would gather in the chapel to chant the service, be it Vespers or Matins. In other words, to sing.
It's tough to say what it sounded like, unfortunately. Bede lived before the standardization of Gregorian chant (which was even slower to take hold in the British Isles), and early insular plainchant had been overtaken by the Gregorian style by the time medieval scribes started using musical notation in the high Middle Ages.
Well, he was a monk, so he would have heard and/or made music in church. Plainchant was a common form of song sung in liturgical masses throughout Europe in the middle ages, of which Gregorian Chant is the most well known.
Plainchant would have been in its most early forms in this time, with many different styles coming out throughout Europe, but sacred singing in the church would have been around since at least the 4th century.